One of the classic design features of MMORPGs is to allow characters to "grow" by gaining new abilities. This winds up becoming the core driver of all gameplay because designers "gate" advancement -- they don't let you have everything at once. You're forced to do a bunch of little things to collect resources that add up over time to allow you to purchase additional abilities, either directly as individual skills or indirectly as abilities derived from rising a level in a character class.
In such games, the question arises: Should there be hard caps on the number of abilities a character can learn?
Hard caps on the number of skills or skill levels a character can posses do a couple of useful things in MMORPGs. One is to simulate the difficulty people have in learning new skills. The other is to increase uniqueness among characters by limiting the skills that can be learned to a subset of all possible skills.
Are those benefits valuable enough to use a hard cap system? Are there other ways of getting these benefits (assuming you agree that they're benefits) than hard caps?
For example, take that first benefit of simulating the difficulty of increasing one's abilities. Instead of a hard cap, you could use an economic approach that makes later skills cost more than skills taken earlier in a character's life. Rather than being an arbitrary number, this would actually be a more accurate way to simulate limits to learning.
There are a couple of obvious ways to do this. One is to rank skills by power and make the more powerful skills cost more to obtain. (A lot of MMORPGs use some form of this approach.) Another would be to treat all skills (and all levels of skills if you have skill levels) as equally difficult, but calculate the learning cost of each new skill based on the number of skills you already have -- the more skills you have, the harder it is to learn a new skill.
As for limiting skills -- usually through a class system -- I believe that casual players are better off with an unlimited skills system than they are with a class-specific skills system.
As evidence, I offer the continuing success of EVE Online and its effectively unlimited skill system. I say "effectively" because while there are a limited number of skills, the amount of time required to learn all the highest levels of all the existing skills is so great that it would take years to do so, by which time the developers will have added new skills to learn. But EVE is still playable by casual players because skills are learned over time, rather than by collecting and cashing in skill points.
Consider the skill Connections (which improves your standing with NPC agents, opening up more quests that you can take). No matter who your character is or what other skills your character knows, no matter when you started playing EVE Online or how many hours a day you play, your character is going to require at least two real-time weeks to learn the fifth and highest level of the Connections skill. Your character learns at the same rate regardless of whether you the player are online in the game or offline, so every player enjoys an absolutely level playing field.
In short, there's no level grind in EVE Online, and thus over time no significant disparity between veteran and casual players. It's a thing of beauty. And there's no reason at all why other games can't figure out some equally creative alternative to mindless grinding.
Why impose hard caps on skills when there are other (and better) ways to regulate the growth of character power?